This paper seeks to raise questions about the growing emphasis on public participation in decision-making in the health service. It examines the case study of lay participation on Local Research Ethics Committees (LRECs'). In the light of contested theoretical conceptions of the value of lay participation and an absence of a centrally defined role this paper examines practice. It uses qualitative evidence collected in 45 semi-structured interviews with committee members and observations of twenty committee meetings. It examines members' own conceptualisations of lay involvement and the contributions they are able to make in meetings as a result of these conceptualisations. It concludes that without better-defined roles for lay members on these committees they do not possess the authority or knowledge to challenge the experts' technical rendering of research.